function signin()
{
var un=/^[a-zA-Z0-9]+$/;
var pas=/^[a-zA-Z]+$/;

x1=document.sf.userName.value;
x2=document.sf.password.value;

if(!un.test(x1))
{
 alert("invalid username");
 document.sf.uname.value="";
 document.sf.uname.focus();
 return false;
}
if(!pas.test(x2))
{
 alert("invalid password");
 document.sf.password.value="";
 document.sf.password.focus();
 return false;
}

}